This invention relates to a power amplifier device, more particularly a transistor power amplifier device.
Various types of transistor power amplifier devices have been proposed. However, in the conventional amplifier devices the collector loss in an output transistor inevitably becomes large in accordance with the output of the amplifier. The large collector loss results in heating the output transistor. As a result, an elaborate cooling means and a high withstand voltage output transistor are required.
These problems will be briefly described with reference to FIG. 1. The prior art power amplifier device shown in FIG. 1 is an OCL (output condencerless) complementary SEPP (complementary single ended push-pull) circuit and comprises an input terminal 11 for receiving an input signal from a preceding stage, such as a low frequency amplifier, not shown, and an amplifier 12 with one input terminal .sym. connected to the input terminal 11 and to the ground through a resistor 13 and the other input terminal .crclbar. connected to the ground through a resistor 14. There are also provided a pair of DC sources 16 and 17 having opposite polarities, and bipolar output transistors 18 and 19, the former being NPN type and the latter PNP type. The transistors 18 and 19 are connected to act complementally with respect to each other and each connected to act as an emitter follower type. The emitter electrodes are commonly connected. The collector electrode of transistor 18 is connected to the positive pole of source 16 via +V.sub.c2 terminal 24 while the collector electrode of transistor 19 is connected to the negative pole of source 17 via -V.sub.c2 terminal 26. The base electrodes of these transistors are commonly connected to the output terminal of the amplifier 12, and the commonly connected emitter electrodes are connected to the grounded juncture between sources 16 and 17 via an output terminal 21 and a load resistor 22. Furthermore, the commonly connected emitter electrodes are connected to the negative terminal of amplifier 12 via a feedback resistor 23. The amplifier 12 is connected to the positive pole of source 16 and the negative pole of source 17 respectively through +V.sub.c1 terminal 23 and -V.sub.c1 terminal 25.
The amplifier circuit shown in FIG. 1 operates as follows. The input signal applied to the input terminal 11 from the preceding stage is amplified by the amplifier 12 and then power-amplified by output transistor 18 or 19 according to the polarity of the input signal and the amplified output is supplied to the common load resistor 22.
In this power amplifier device, the source 16 or 17 is required to supply voltage equal to the sum of the maximum output voltage appearing at the output terminal 21 and the voltage loss of the transistor 18 or 19. Accordingly, the voltage V.sub.CE appearing across the collector and emitter electrodes of transistor 18 or 19 is equal to the difference between the source voltage and the output voltage appearing at the output terminal 21, and a power equal to the product of this voltage V.sub.CE and the output current generates a collector loss P.sub.c. In the prior art power amplifier device described above the collector-emitter voltage V.sub.CE is different between the normal output and the maximum output. This voltage occupies a substantial portion of the source voltage in the presence of an input signal, and under these conditions, when the output current is increased the power loss of the output transistor 18 or 19 becomes substantial. For example, the efficiency of the prior art A class power amplifier is about 40 to 50% and that of the B class power amplifier is about 60 to 70%. From this fact, it can be readily understood that the power loss of the output transistor is large. Accordingly, the power loss (collector loss) and hence heating of the output transistors increases as the output increases. For this reason, the prior art power amplifier device requires to use expensive high power transistors or to provide elaborate cooling means.
Moreover, as the value of the collector-emitter voltage V.sub.CE varies greatly with the magnitude of the input signal it is impossible to maintain the linearity between the variation in the base current and .DELTA.V.sub.CE /.DELTA.I.sub.c, where I.sub.c represents the collector current. Furthermore, as the amplitude varies up to the saturation range, it is liable to form a switching distortion.
It is also necessary to increase the breakdown voltage across the collector and emitter electrodes of the output transistors beyond twice or more of the source voltage by taking into consideration the fact that an abnormal transient voltage is impressed across the collector and emitter electrodes at the time of cutting off which is caused by an inductive load. This also requires expensive output transistors.
As above described, in the prior art transistor power amplifier device variation in the collector-emitter voltage V.sub.CE of the output transistors causes various problems described above. This is especially true in high output power amplifier devices and A class power amplifier devices.
A pulse amplifier has been developed as a power amplifier having a small collector loss and a high efficiency of the order of 90% but this type of the power amplifier requires a special modulation circuit (PCM or PWM) and a carrier wave filter thereby complicating the construction and increasing the cost. Moreover, the linearity and fidelity of the component parts have a large influence upon the characteristics of the power amplifier.